1. Field of the Invention
The present invention relates to a recording head adjustment method and an image recording apparatus, and more particularly, to technology for adjusting a recording head having a structure in which a plurality of sub-heads are joined together.
2. Description of the Related Art
In an inkjet recording apparatus which is a generic image recording apparatus and which forms a desired image on a recording medium by means of an inkjet method, technology is used in which a long line type inkjet head corresponding to the entire width of a recording medium is formed by joining together, in the width direction of the recording medium, short sub-heads which are shorter than the entire width of the recording medium. In an inkjet head having a structure of this kind, installation error of the respective sub-heads produces relative error (step difference) in the depositing positions of the ejected droplets, thus affecting image quality. In order to perform high-quality image recording by eliminating such a step difference, the relative positions of the sub-heads would be better off being adjusted accurately so as to remove step differences between the sub-heads.
However, it is extremely difficult to adjust the relative positions of a plurality of sub-heads accurately by a mechanical method. As a method of correcting relative positional error between sub-heads in the direction of conveyance of the recording medium, it is desirable to use an electrical adjustment method in which the depositing position error in the conveyance direction of the recording medium is measured for each sub-head, the difference in ejection timing corresponding to the depositing position error is determined, and the ejection timing of each sub-head is altered respectively in accordance with this difference in the ejection timing.
Japanese Patent Application Publication No. 2009-51066 discloses technology which determines depositing position errors with respect to a reference position in the sub-scanning direction for each small head of a large head which is constituted by a plurality of small heads having a plurality of ejection units, and optimizes the reading address of print data and the printing timing for each small head.
A method of correcting step differences between sub-heads by an electrical method in an inkjet head having a structure formed by joining together a plurality of sub-heads is now described.
FIG. 11 is a partial enlarged diagram of a pattern 300 formed in order to measure the amount of step difference between sub-heads, and depicts an enlarged view of the portion formed by an overlapping portion (joint portion) between sub-heads. The vertical direction in FIG. 11 is the direction of movement of the recording medium, and the left/right direction is the direction of alignment of the sub-heads. The term “overlapping portion between sub-heads” means a portion where there is a mixture of nozzles belonging to one sub-head and nozzles belonging to another sub-head in a projected nozzle group in which the nozzles belonging to the two mutually adjacent sub-heads are projected so as to align in a direction substantially perpendicular to the direction of movement of the recording medium. In other words, droplets ejected from an overlapping portion between sub-heads include droplets ejected from different sub-heads, which are deposited at mutually adjacent positions in the direction substantially perpendicular to the direction of movement of the recording medium.
The pattern 300 shown in FIG. 11 is formed by an overlapping portion between an ith sub-head and an (i+1)th sub-head, and includes a group of horizontal lines 302 (302-1, 302-2, . . . , 302-7) formed by the ith sub-head and a group of horizontal lines 304 (304-1, 304-2, . . . , 304-7) formed by the (i+1)th sub-head, the lines of the respective groups being arranged in alternating fashion. In other words, a pattern 300 in which horizontal lines 302-1 to 302-7 constituting the horizontal line group 302 and horizontal lines 304-1 to 304-7 constituting the horizontal line group 304 are arranged alternately at a uniform pitch in the direction of conveyance of the recording medium is formed, by driving the ith sub-head at a prescribed drive cycle to form a group of horizontal lines 302 arranged at a prescribed pitch in the direction of movement of the recording medium, and also driving the (i+1)th sub-head at the same drive cycle as the ith sub-head and at a time interval of ½ of the drive cycle of the ith sub-head, from the driving timing of the ith sub-head.
As shown in FIG. 11, in the pattern 300 which is actually formed, the pitch P1 between the horizontal line 302-1 formed by the ith sub-head and the horizontal line 304-1 formed by the (i+1)th sub-head is smaller than the pitch P2 between the horizontal line 304-1 formed by the (i+1)th sub-head and the horizontal line 302-2 formed by the ith sub-head. In other words, the position of the (i+1)th sub-head in the direction of movement of the recording medium is displaced toward the downstream side (the upper side in FIG. 11). Cases such as this are corrected by delaying the drive timing of the (i+1)th sub-head in accordance with the amount of displacement of the depositing positions.
FIG. 12 shows a pattern 300′ formed by using an inkjet head in which the drive timings of respective sub-heads have been adjusted as described above. The pattern 300′ shown in FIG. 12 is an enlarged view of a portion formed by an overlapping portion between sub-heads. In the horizontal lines which constitute the pattern 300′ shown in FIG. 12, there are regions where bending occurs, and therefore the measurement values vary depending on which region is the subject of measurement and it is difficult to ascertain the pitch between the horizontal lines accurately. For example, looking at the first horizontal line 302′-1 in FIG. 12 and the second horizontal line from the top 304′-1, the pitch at the left end is P1, whereas the pitch at the right end is P1′ (>P1), and hence the measurement result changes with the measurement position. If accurate measurement results are not obtained in this way, then it is not possible to evaluate the step differences between sub-heads accurately, and therefore it is difficult to eliminate these step differences. If step differences of this kind are not adjusted accurately, then the image quality declines as a result. On the other hand, it is possible to adjust for step differences with a certain degree of accuracy by repeating steps of measurement and adjustment, but the amount of work involved in the adjustment process increases dramatically.
Furthermore, although the nozzle surface (liquid ejection surface) in which the nozzle holes are formed has a prescribed flatness in each individual sub-head, the nozzle surface bends slightly during processing for forming the nozzle holes and during assembly for fixing the sub-heads to a housing. The example shown in FIG. 13A is a case where a nozzle surface 312 is bent in the direction of movement of the recording medium. The nozzle surface 312 receives pressure from the interior of the head 310 and therefore the bending of the nozzle surface 312 is convex toward the direction of ejection of the liquid. In this case, the direction of ejection of the liquid in the edge portions 314 of the sub-head (labeled with reference numeral A) is the normal direction to the curved surface and therefore displacement occurs in the depositing positions in the direction of movement of the recording medium. The positions labeled with reference numerals 320 and 322 in FIG. 13A are the original depositing positions, and the positions labeled with reference numerals 320′, 322′ are the actual depositing positions in which positional displacement has occurred. On the other hand, in substantially the central portion of the sub-head, since the liquid is ejected in a direction perpendicular to the recording medium 316, then the ejected droplets land in the original depositing position 324.
Depositing position displacement caused by bending of the nozzle surface 312 affects the step difference in the overlapping portion between sub-heads. FIG. 13B is an enlarged diagram showing an enlarged schematic view of the overlapping portion between the ith sub-head 310-i and the (i+1)th sub-head 310-(i+1). Droplets ejected from the −Y side nozzles 330A in the nozzle arrangement in the column direction (an oblique direction forming a prescribed angle with respect to the Y direction) land further in the −Y direction (indicated by the upward pointing arrows in FIG. 13B), and droplets ejection from the +Y side nozzles 330B land further in the +Y direction (indicated by the downward pointing arrows in FIG. 13B). On the other hand, the displacement in the depositing positions of droplets ejected from the nozzles 330C in substantially the central portion in the column direction of the nozzle arrangement is relatively small.
Furthermore, since the extent of bending is different in the sub-head 310-i and the sub-head 310-(i+1), then the extent of displacement of the depositing positions is different for each sub-head. The length of an arrow shown in FIG. 13B represents the amount of depositing position displacement, and it can be seen that the amount of depositing position displacement is smaller in the sub-head 310-(i+1) than in the sub-head 310-i. In particular, there is marked depositing position displacement between droplets ejected from nozzles on the −Y direction side in the nozzle arrangement in the column direction and droplets ejected from nozzles on the +Y direction side.
In the technology disclosed in Japanese Patent Application Publication No. 2009-51066, although it is conceivable that bending may occur in the horizontal lines in the portions formed by the overlapping portions between sub-heads, Japanese Patent Application Publication No. 2009-51066 does not disclose a method for eliminating depositing position error in the droplets formed by overlapping portions between sub-heads. Consequently, even if the technology disclosed in Japanese Patent Application Publication No. 2009-51066 is used, it is difficult to ascertain the step differences between sub-heads accurately, and to correct the step differences between sub-heads completely. Furthermore, Japanese Patent Application Publication No. 2009-51066 makes no mention of focusing on the depositing position error of the droplets caused by bending of the nozzle surface and does not describe technology for eradicating a depositing position error of this kind.